networkd: Routes should take the gateway into account

networkd: Routes should take the gateway into account

Otherwise, changing the default gateway doesn't purge old gateway routes
left on the system during daemon restart. This also fixes removing other
foreign gateway routes that don't match the expected configuration.

Tested:
    Changed gateway addresses prior to the patch and they lingered on
    the system during each reconfiguration. Applied this patch and
    reconfigured gateways and other routes multiple times and it removed
    the foreign routes that had gateways that didn't match.

Signed-off-by: William A. Kennington III <william@wkennington.com>